In many applications, electric discharge machining is used for the mass production of parts which must be machined to close tolerances. There are several factors which affect the repeatability of accuracy of machining by the EDM process including the process parameters of electrode size, spark energy, spark current, gap length, gap voltage and the wire guide used to direct the electrode to a workpiece. For example, when the EDM process is used to machine a small hole through a thin plate, the small dimensions of the wire electrode require a support system which is sufficiently accurate to support the electrode in all directions. Also, mechanical or electrical vibrations in the electrode system can result in variation in hole diameters. In the case of wire guides for directing electrodes with respect to an orifice plate for fuel injectors, for example, the electrode, even though closely guided, may have a biasing force acting thereon that can bend the wire between the wire feed and the wire guide which positions the wire electrode with respect to the work piece.
The biasing force acting on the wire electrode should be maintained throughout the life of the electrode guide and should be at a level which will prevent wire bending. Furthermore, the wire electrode should have a zero clearance between the wire guide at the exit from the wire guide to form accurately positioned holes in the workpiece.
Accordingly, there is a need for obtaining improved uniformity and accuracy in guiding an electrode wire in EDM apparatus. Further, it should be achieved without affecting the various other parameters involved in improving accuracy of the EDM process.
U.S. Ser. No. 039,065, filed Apr. 16, 1987, entitled Wire Guide for Electric Discharge Machine, discloses one arrangement for imposing a spring force on a wire electrode to control the position of a wire electrode with respect to a work piece. The wire guide housing engages the wire electrode throughout the length of the housing. This increases the friction force on the wire as it is advanced with respect to the wire guide.